The invention relates to an illumination system for a photographic enlarging or other copying apparatus.
In enlarging or other copying apparatus in which the image original to be copied is acted on by diffuse light, light-diffusing means is generally necessary to achieve this type of illumination. So-called light-mixing ducts are frequently used in this connection, the inner surfaces of which reflect, or effect diffuse reflection, and which are generally provided, at one position at least, with light-diffusing means occupying completely or partially the cross-sectional area of the light-mixing duct, for example, in the form of a diffuser screen extending over the cross-sectional area. The diffuser screen may be, for example, a ground glass screen. Illumination systems are known in which light-diffusing means in the form of a diffuser screen is provided in the cross-sectional area of such a light-mixing duct either at the end where the light enters or the end where the light leaves.
It is especially important to get good light-mixing with color copiers of certain kinds, that is, when the proportion of the different primary colors in the copying beam is varied by partially inserting a filter into the beam.
Diffuser screens made of plastics material, which have a very good light-mixing action and are easy to fashion, are frequently used in conjunction with light-mixing ducts. Diffuser screens of this kind, however, can be used at the light-entering end of a light-mixing duct only if they are arranged a sufficient distance away from the light source or if the light source only has a low output.